Modern military aircraft employ electronic warfare systems as part of their offensive and defensive capabilities. Many of those systems emit RF signals which are an invisible form of energy that travels through space. Radar systems use RF emissions to locate and track opposing aircraft and some radar systems are incorporated within missiles to assist in the self-guided propulsion of a missile to its target. From a defensive aspect, an electronic warfare search receiver is used to detect those RF emissions. The receiver searches the range of frequencies in which those RF emissions are likely to occur (the RF spectrum) to detect and analyze the nature of the RF signals. By determining the characteristics of the signals received by this equipment, the defender will know the nature of the threat and, for example, will know if a radar guided missile has "locked on" to the defenders aircraft. These systems are used in friendly as well as in unfriendly aircraft. In a tactical or strategic environment, the number of aircraft and, hence, the density and diversity of the emissions in the RF spectrum is quite large and is expected to increase. Existing detection and monitoring equipment that use wide band search receivers will find the RF emissions difficult or impossible to successfully monitor in such an environment. For example, some existing wide band receiver designs employ a threshold detector that requires the incoming signal to attain a certain amplitude before it is recognized as a true signal apart from the ordinary RF background noise. These receivers are incapable of detecting two different RF pulses that occur simultaneously. With the existing design, it is entirely possible that the first RF pulse received will effectively prevent detection of a second RF pulse, from another emitter, occurring during the presence of the first pulse. The first emission source may be identified, but the second source is in effect masked. In a tactical environment, the failure to detect the existence of a second emission source is a disadvantage.
In traditional Electronic Warfare/Electronic Support Measures (EW/ESM) receivers, techniques are used in which the receiver's video output is digitized. Digitization occurs when the amplitude of the input signal exceeds a predetermined threshold level. After the threshold has been crossed, the signal parameters for that signal are digitized. However, if a second signal occurs before the first signal drops below the threshold, then the second signal will not be detected. This allows a continuous wave or long pulse width signal to prevent detection of subsequent signals occurring simultaneously with tile first signal, even if the subsequent signals are significantly larger in amplitude than the first signal.
It is unlikely that a single receiver type will be capable of meeting all offensive or defensive threat detection and analysis requirements dictated by the future electronic warfare environment. Instead, a set of search and analysis receivers with complementary capabilities are likely to be required to meet future demands. Trade-offs between probability of intercept, bandwidth, simultaneous signal resolution, sensitivity and receiver complexity are necessary. Simultaneous signal resolution is an important requirement that needs to be addressed in order to reduce the risk that an enemy radar signal will go undetected. Various techniques have been proposed to detect simultaneous signal conditions and suppress invalid data. One such system uses a dispersive delay line to separate signals, such as shown in U.S. Pat. No. 3,939,411. With this background it is desirable to provide a wide band receiver that has a high probability of detecting simultaneous signals and that can also measure in real time the phase, frequency, time of arrival, pulse modulation, pulse width and amplitude of each signal.